This invention relates to the selective aliphatic halogenation of the .alpha. carbon of alkylbenzenes and their aromatically-substituted derivatives. More specifically, it relates to the .alpha.-chlorination of aromatically-substituted derivatives of cumene.
It is known that halogenation of an aliphatic hydrocarbon moiety is most often a free radical process initiated by light or a free radical initiator. Generally, halogenation of aliphatic hydrocarbons is a poor method of synthesis because substitution of the halogen is largely indiscriminate. For example, in the chlorination of ethylbenzene or longer-chain alkylbenzenes, both .alpha. and .beta. chlorination occur because disubstitution rates are nearly equal to monosubstitution rates. Bromination of higher alkyl side chains is more selective than an analogous chlorination and, therefore, yields a larger proportion of .alpha.-substituted products. Alternatively, selective halogenation of the .alpha. carbon can be increased by supplying a large excess of the alkylbenzene. Unfortunately, this excess lowers the conversion of the alkylbenzene to the .alpha.-halogenated product. See, e.g., Survey of Organic Syntheses, Vol. I, C. A. Buehler and D. E. Pearson (1970). As a result of the aforementioned indiscriminate substitution, prior methods of selectively halogenating the aliphatic chains of alkylbenzenes required alkylbenzene recycle streams for the excess hydrocarbon, and required expensive separation steps due to the low conversion to product.
In U.S. Pat. No. 2,193,823, a method is taught for low yield .alpha.-chlorination of ethylpentachlorobenzene by contacting the same in the liquid phase at elevated temperature with chlorine gas in the presence of the light of an ordinary electric light bulb to produce .alpha.-chloroethylpentachlorobenzene. The desired product must be separated from the reaction mixture by treating the mixture chemically to remove residual chlorine, followed by two vacuum distillation steps.
In U.S. Pat. No. 4,133,837, the preferential aliphatic halogenation (excluding chlorination) of one or more isomers in a mixture of ar-substituted alkylbenzenes was disclosed. Heating the crude product mixture under vacuum is taught as a method of separating out the chlorobenzene solvent.
Heretofore, a process for the selective chlorination of the .alpha. carbon of an alkyl side chain of an aromatically-substituted cumene derivative has not been disclosed.